Aircraft electric power generation and distribution systems typically utilize multiple generators driven by the aircraft engines and coupled by hundreds of feet of power feeders to supply the various electrical loads distributed throughout the aircraft with electrical energy. Safety concerns about the risk of fire and loss of electrical power resulting from a short circuit on a main power feeder on the aircraft have driven engineers to develop very sophisticated protection systems to guard against such faults. Because at least some of the electric current generated by the generator flows into the short circuit, depending on the impedance of the fault, a difference exists between the current generated by the generator and that which is actually delivered to the aircraft loads. Modern protection systems sense this differential current flow, and allow proper isolation of the fault before damage is sustained to the aircraft.
The most common method of detecting these differential current faults is to measure current flow at two or more points in the power system. These measurements are then compared to determine if there are any differences in the current magnitude. Since, in the absence of any fault, there is no current loss along a feeder, any difference in current magnitude is the result of current flowing via unintended paths. These unintended paths represent faults on the system and must be isolated. Conventional AC power systems use well matched AC transformers to measure the current flow out of the source and into the loads. The output of these two sets of transformers are then directly compared to determine the presence of a fault. Examples of such systems may be found in U.S. Pat. Nos. 4,173,774; 4,321,645; and 5,047,890 assigned to the assignee of the instant invention.
Some modem aircraft are being developed utilizing equipment requiring a `hybrid` electric power system comprising AC, low voltage DC (typically 28 Vdc), and/or high voltage DC (typically 270 Vdc) power generation. The specific components vary among differing configurations, but typically include an AC generator whose output is coupled both to utilization equipment for supplying AC power thereto and to some type of AC to DC conversion equipment, such as a rectifier filter, or a step-up or step-down transformer rectifier unit (TRU). The output of the AC to DC conversion equipment then supplies DC power to utilization equipment requiring such power.
A problem with this type of system, illustrated in simplified single line form in FIG. 1, concerns providing adequate protection from faults allowing current to flow along unintended paths, or differential current faults. Conventional methods of protection sense the AC current generated by the generator 100 and that which is delivered to the loads to determine if a differential current fault exists on the AC system 102, and sense the DC current converted by the AC to DC conversion equipment 104 and that which is delivered to the DC loads to determine if a differential current fault exists on the DC system 106. In addition to this type of protection system requiring at least four (4) points of sensing current 108, 110, 112, and 114 and at least two separate comparison circuits 116 and 118, it also leaves the AC to DC conversion stage 104 unprotected, i.e. a differential current fault caused by a short circuit within the AC to DC conversion equipment itself is not within the zone protected by the AC protection 102, nor within the zone protected by the DC protection 106. Sustained operation with such a fault within the conversion equipment 104 may result in serious damage to the equipment and potentially the aircraft itself.
Protection for internal AC to DC conversion equipment faults has typically been accomplished by a separate circuit which monitors the peak-to-peak AC voltage of the DC output. Upon the occurrence of a fault, such as a shorted rectifying diode, the output ripple of the DC voltage would increase, and the fault would be detected. This sensing may not detect many rectifier diode open and some shorted diode failures, however, because the sensing threshold, which is set to coordinate with the expected normal waveform disturbances caused by load or source variations, is necessarily set too high. A lowering of the threshold, while allowing more of these types of faults to be detected, will also result in false indications of a fault during normal, unfaulted conditions as various loads are connected and disconnected to the system.
The instant invention overcomes these problems by providing a system of protection which minimizes the number of components and circuitry required to sense both distribution and internal conversion failures while maximizing the zone of protection to include both AC generation and distribution equipment and AC to DC conversion and distribution equipment.